1. Field of the Invention
The invention lies in the field of electronics and in particular relates to electrical circuits for producing a simmer current to high output flash lamps such as those made of Xenon and other ionization tubes where a certain population or degree of ionization is desired to be maintained. More specifically, the invention relates to electrical circuits for supplying a low and steady current to maintain ionization in a flash lamp load that customarily has a negative resistance at that level of current.
2. Description of Related Art
Pulsed lasers are commonly pumped to a population inversion with a high intensity light source. To provide this light, most pulsed lasers use flash lamps such as the standard Xenon flash lamp. In operation, the gas in the flash lamp tube must first be ionized with a high voltage before a current can be initiated. For Xenon, a voltage of about 40,000 volts is required to initiate a current. After current initiation, the voltage drop across the tube decreases rapidly to a few hundred volts. Thereafter, with a small, steady current through the flash lamp tube of approximately 100 milliamps, ionization will be maintained in a Xenon tube. This small, steady current is termed a "simmer current." When a high intensity flash pulse is desired, a high power electrical pulse is applied to the flash lamp, which in turn reliably produces the high intensity flash which then pumps the laser gain material.
Electrical characteristics of a flash lamp are illustrated in the FIG. 1 graph of voltage vs. current where three independent curves are drawn. In FIG. 1, the straight line curve labeled PS indicates the load line of a Power Supply for a simmer current to a flash lamp. PS indicates that at maximum voltage the power supply delivers minimum current, and at minimum voltage the supply would yield maximum current. Straight line curve RL is indicative of a typical Resistor Load that one might expect in any circuit. As the voltage increases so to does the current flow; at maximum voltage, we get maximum current and at minimum voltage, we get minimum current. On the other hand the unconventional or unusual Flash Lamp voltage vs. current load line is indicated in curve FL. It will be observed that a large voltage must be applied to initiate the current. Following current initiation, voltage decreases rapidly while current increases, resulting in a negative slope. For maximum operational efficiency, it is suggested to simmer a flash lamp at a voltage and current at an Optimum Point indicated as OP in FIG. 1.
Maintaining this operational point, however, creates a stability problem due to the negative resistance of the flash lamp. To address the stability problem, prior art teaches that a large resistor R need be coupled in series with a voltage source VS as illustrated in FIG. 2 to limit essentially infinite current flow at low voltage past the OP. The result is a circuit that works, but the power efficiency of such a prior art circuit is only 5 to 10%. A large amount of waste heat is produced in the resistor R, and a relatively large and bulky package is required for the physically large high power components.
The invention disclosed herein resolves the foregoing long standing limitations and prior art problems with a simmer current circuit in a unique and novel manner. The invention discloses an electrical circuit that provides a steady current to a negative resistance load such as a flash lamp that pumps a pulsed laser. Such a steady current can simmer the flash lamp so that it remains ionized throughout a series of pulses, while at the same time performing at a substantially higher efficiency (70 to 80%) in converting electrical energy to current through the flash lamp. By such means, the invention offers less heat generation, less loss of energy, less expense, longer life, as well as a much smaller, lighter and compact package of components.